The invention relates to a method for preparing essentially color-free, stabilized polymers of conjugated dienes or vinyl aromatic hydrocarbons or copolymers or conjugated dienes and vinyl aromatic hydrocarbons. More particularly, the invention relates to a method for preparing essentially color-free, stabilized polymers of conjugated dienes or vinyl aromatic hydrocarbons or copolymers of conjugated dienes and vinyl aromatic hydrocarbons which involves the use of boric acid as a treating agent to prevent or substantially reduce the formation of color in the polymer or copolymer.
Color-free polymers and copolymers are of increasing interest. Thus, for example, color-free block copolymers of conjugated dienes and vinyl aromatic hydrocarbons, particularly those having high contents of vinyl aromatic hydrocarbon, have enjoyed increased usage in applications such as garment hangers, food containers, food packaging materials, beverage cups, medical sundries and the like.
A number of block copolymers of the aforementioned type and methods for their preparation have been described in the prior art as illustrated by U.S. Pat. Nos. 3,639,517 to Kitchen et al; 4,067,942 to Wilson; 4,073,831 to Tabana et al; 4,086,298 to Fahrback et al; 4,089,913 to Miki et al; 4,117,545 to Fahrback et al; 4,195,136 to Sato et al and 4,208,356 to Fukawa et al and U.S. Defensive Publications T953,011; T954,012; T959,011 and T967,004 to Smith.
While the block copolymers described in the above patents and publications exhibit many advantageous properties, they have been found to suffer from a significant disadvantage which materially detracts from their usefulness in applications where lack of color is a critical property such as in food containers, food packaging materials, beverage cups and the like. Thus, it has been found that block copolymers of conjugated dienes and vinyl aromatic hydrocarbons, particularly those having high contents of vinyl aromatic hydrocarbons, often exhibit a yellowish cast or tint as prepared, or develop such a yellowish cast or tint under conditions of storage and use. In fact, applicant has surprisingly discovered that this tendency to exhibit or develop a yellowish cast or tint is not limited to block copolymers containing high contents of vinyl aromatic hydrocarbons, but also extends to block copolymers having low contents of vinyl aromatic hydrocarbon, random copolymers of conjugated dienes and vinyl aromatic hydrocarbons and even homopolymers of conjugated dienes or vinyl aromatic hydrocarbons.
Accordingly, the discovery of an uncomplicated method for preparing stabilized polymers of conjugated dienes or vinyl aromatic hydrocarbons or copolymers of conjugated dienes and vinyl aromatic hydrocarbons which are essentially color-free as prepared, and which do not develop undesirable color under normal conditions of strorage and use, is highly desirable and would represent a development of major significance.